Catalytic hydroprocessing refers to petroleum refining processes in which a hydrocarbon feedstock is brought into contact with hydrogen and a catalyst, at a higher temperature and pressure, for the purpose of removing undesirable impurities and/or converting the feedstock to an improved product.
Hydroisomerization is an important refining process used to catalytically dewax hydrocarbon feedstocks to improve the low temperature properties of lubricating base oil and fuel fractions. Catalytic dewaxing removes long chain n-paraffins from the feedstock which, if otherwise not removed, have a negative impact on the pour and cloud points of the fractions; however, dewaxing also lowers the Viscosity Index (VI) of the base oil fraction as well. A high VI is necessary to provide the base oil with temperature range insensitivity, meaning the base oil is capable of providing lubricity at both low and high temperatures.
Refiners operating a catalytic dewaxing unit wish to maximize yields and meet the target product specifications (VI, pour point), while minimizing the reactor temperature (which corresponds to costly hydrogen consumption and VI reduction at higher temperatures) and light ends (C4−) production.
Lubricating base oil distillate fractions are generally referred to as neutrals, e.g. heavy neutral, medium neutral and light neutral. The American Petroleum Institute (API) classifies finished lubricating base oils into groups. API Group II base oils have a saturates content of 90 wt. % or greater, a sulfur content of not more than 0.03 wt. % and a VI of greater than 80 but less than 120. API Group III base oils are the same as Group II base oils except the VI is at least 120.
Generally, conventional hydroisomerization catalysts are composed of (1) at least one molecular sieve suitable for isomerizing long-chain n-paraffins; (2) a binding material (also referred to as the “support material”) such as alumina, titania, silica, etc; and (3) one or more active hydrogenation/dehydrogenation metals selected from Groups 6 and 8-10 of the Periodic Table, particularly platinum and palladium.
There are two broad classes of reactions that occur in the hydroisomerization process. The first class of reactions involves hydrogenation/dehydrogenation, in which aromatic impurities are removed from the feedstock by saturation. The second class of reactions involves isomerization, in which long chain n-paraffins are isomerized to their branched counterparts.
Hydroisomerization catalysts are bifunctional: hydrotreating is facilitated by the hydrogenation function provided by the metal components, and the isomerization reaction is facilitated by the acidic molecular sieve components. Both reactions need the presence of high pressure hydrogen.
During dewaxing, the wax molecules (straight chain paraffins) undergo series of hydroconversions: hydroisomerization, redistribution of branches and secondary hydroisomerization. The process starts with increasing the degree of branching through consecutive hydroisomerization accompanied by redistribution of branches. When the degree of branching increases, the probability of cracking increases, which will result in formation of fuels and decrease in lube yield. The improvement in porosity of the hydroisomerization catalyst favors minimizing the formation of hydroisomerization transition species by lowering the residence time and by increasing the sweeping efficiency, thus decreases the probability of cracking. This leads to the enhancement in the hydroisomerization performance.
Accordingly, there is a current need for a hydroisomerization catalyst that exhibits a higher degree of hydrogen efficiency and greater product yield and quality, as compared to conventional hydroisomerization catalysts.